Recent years have seen the development of biometric authentication techniques for authenticating a user of an apparatus or system by utilizing a biometric image representing the user's biometric information such as a fingerprint or a palm print. A biometric authentication device using such biometric authentication techniques acquires, as an input biometric image, a biometric image representing, for example, the biometric information of the user who is to use the biometric authentication device. Then, the biometric authentication device compares the user's input biometric information represented in the input biometric image against registered biometric information of preregistered users represented in registered images. If it is determined as a result of the comparison that the input biometric information matches the registered biometric information, the biometric authentication device authenticates the user as being a legitimate registered user. Then, the biometric authentication device permits the thus authenticated user to use the biometric authentication device or some other device connected to the biometric authentication device.
In the use of such biometric authentication techniques, it is desirable that the characteristic features of the biometric information be clearly captured in the input biometric image as well as the registered biometric image in order for the user to be authenticated with high accuracy. However, depending on the condition of the input biometric information or on the operation performed by the user on the device for entering the biometric information, a locally or globally blurred input biometric image or registered biometric image may result.
For example, when the biometric authentication device acquires a fingerprint image of a particular finger of the user as the biometric image representing the biometric information, if the surface of that particular finger is moistened with sweat, a fingerprint image containing a blurred region with the sweat building up between the ridges of the finger will be captured by the biometric authentication device. Further, if the user presses the finger too hard onto the fingerprint sensor, the height of the ridges relative to the valleys is reduced in the pressed portion; in this case also, a fingerprint image containing a blurred region will be captured by the biometric authentication device. Since the biometric authentication device extracts, for example, the end points or branch points of the ridges as the feature points for matching, it is difficult to accurately extract the feature points from such a blurred region. As a result, in such a blurred region, the biometric authentication device becomes unable to accurately examine the degree of matching between the feature points of the input fingerprint image and the feature points of the preregistered fingerprint image, and the accuracy of matching thus degrades. In view of this, techniques have been developed that detect such a blurred region from the image representing the input biometric information or correct such a blurred region.
For example, patent document 1 discloses an authentication device which successively samples a plurality of sub-images by an image sampling means from a subject that is moving relative to the image sampling means. Then, the authentication device removes images of non-moving patterns contained in the plurality of sub-images, thereby removing from the sub-images such images as sebum, sweat, vapor, etc., adhering to the sensor surface.
Patent document 2 discloses a striped pattern extraction system which extracts from an image a predetermined number of local direction angles of the line segments forming the striped pattern and computes the reliability evaluation value of each of the extracted direction angles. Then, the striped pattern extraction system suppresses the reliability evaluation value for any pixel for which the number of parallel lines is not larger than a prescribed condition, thereby making it possible to correct blurred ridges due to streaks, solid shading, etc.
Patent document 3 discloses a finger/palm print image processing system which divides the finger/palm print image into a plurality of sub-regions and applies a frequency transform to each sub-region to obtain a plurality of frequency components representing the sub-region. Then, the finger/palm print image processing system determines whether the sub-region is a region that contains a fine structure by checking whether there is any difference between the frequency-transformed result of the center portion of the sub-region and the frequency-transformed result of the sub-region including its peripheral portion. The finger/palm print image processing system can thus correctly identify the finger/palm print region even when the finger/palm print region contains a blurred portion.
Patent document 4 discloses an input fingerprint condition identifying device which determines whether a fingerprint formed from ridges and valleys exists in each of the regions into which the input fingerprint image has been split and identifies the skin surface condition in each split region. In particular, for each split region, the input fingerprint condition identifying device determines whether the split region is in a moistened condition or in a dry condition, based on the proportion of pixels representing ridges and the proportion of pixels representing valleys or background. Then, the input fingerprint condition identifying device identifies the fingerprint condition, based on such parameters as the proportions of the moistened regions and the dry regions to the entire area.
Patent document 1: International Publication Pamphlet No. WO2004-26139
Patent document 2: Japanese Laid-open Patent Publication No. 2005-10842
Patent document 3: International Publication Pamphlet No. WO2005-86091
Patent document 4: Japanese Laid-open Patent Publication No. 2002-298126